diff --git a/DESCRIPTION b/DESCRIPTION
index 8a06e6f56..6fecc73c4 100755
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -21,7 +21,8 @@ Description: This package introduces a common data structure for epidemiological
     work with revisions to these data sets over time, and offers associated
     utilities to perform basic signal processing tasks.
 License: MIT + file LICENSE
-Imports: 
+Imports:
+    cli,
     data.table,
     dplyr (>= 1.0.0),
     fabletools,
@@ -48,7 +49,7 @@ Suggests:
     knitr,
     outbreaks,
     rmarkdown,
-    testthat (>= 3.0.0),
+    testthat (>= 3.1.5),
     waldo (>= 0.3.1),
     withr
 VignetteBuilder: 
diff --git a/NAMESPACE b/NAMESPACE
index 10847e6c9..065302d75 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -84,6 +84,7 @@ importFrom(dplyr,ungroup)
 importFrom(lubridate,days)
 importFrom(lubridate,weeks)
 importFrom(magrittr,"%>%")
+importFrom(purrr,map_lgl)
 importFrom(rlang,"!!!")
 importFrom(rlang,"!!")
 importFrom(rlang,.data)
@@ -91,6 +92,7 @@ importFrom(rlang,.env)
 importFrom(rlang,arg_match)
 importFrom(rlang,enquo)
 importFrom(rlang,enquos)
+importFrom(rlang,is_missing)
 importFrom(rlang,is_quosure)
 importFrom(rlang,quo_is_missing)
 importFrom(rlang,sym)
@@ -101,3 +103,4 @@ importFrom(tidyr,unnest)
 importFrom(tidyselect,eval_select)
 importFrom(tidyselect,starts_with)
 importFrom(tsibble,as_tsibble)
+importFrom(utils,tail)
diff --git a/R/grouped_epi_archive.R b/R/grouped_epi_archive.R
index 1c6bd3110..76b079a4a 100644
--- a/R/grouped_epi_archive.R
+++ b/R/grouped_epi_archive.R
@@ -220,6 +220,11 @@ grouped_epi_archive =
               ref_time_values = sort(ref_time_values)
             }
             
+            # Check that `f` takes enough args
+            if (!missing(f) && is.function(f)) {
+              assert_sufficient_f_args(f, ...)
+            }
+
             # Validate and pre-process `before`:
             if (missing(before)) {
               Abort("`before` is required (and must be passed by name);
diff --git a/R/slide.R b/R/slide.R
index ea2b93cba..d8d6becba 100644
--- a/R/slide.R
+++ b/R/slide.R
@@ -155,7 +155,12 @@ epi_slide = function(x, f, ..., before, after, ref_time_values,
 
   # Check we have an `epi_df` object
   if (!inherits(x, "epi_df")) Abort("`x` must be of class `epi_df`.")
-  
+
+  # Check that `f` takes enough args
+  if (!missing(f) && is.function(f)) {
+    assert_sufficient_f_args(f, ...)
+  }
+
   # Arrange by increasing time_value
   x = arrange(x, time_value)
   
diff --git a/R/utils.R b/R/utils.R
index b398ff5b6..d17f05d4e 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -100,6 +100,87 @@ paste_lines = function(lines) {
 Abort = function(msg, ...) rlang::abort(break_str(msg, init = "Error: "), ...)
 Warn = function(msg, ...) rlang::warn(break_str(msg, init = "Warning: "), ...)
 
+#' Assert that a sliding computation function takes enough args
+#'
+#' @param f Function; specifies a computation to slide over an `epi_df` or
+#'   `epi_archive` in `epi_slide` or `epix_slide`.
+#' @param ... Dots that will be forwarded to `f` from the dots of `epi_slide` or
+#'   `epix_slide`.
+#'
+#' @importFrom rlang is_missing
+#' @importFrom purrr map_lgl
+#' @importFrom utils tail
+#'
+#' @noRd
+assert_sufficient_f_args <- function(f, ...) {
+  mandatory_f_args_labels <- c("window data", "group key")
+  n_mandatory_f_args <- length(mandatory_f_args_labels)
+  args = formals(args(f))
+  args_names = names(args)
+  # Remove named arguments forwarded from `epi[x]_slide`'s `...`:
+  forwarded_dots_names = names(rlang::call_match(dots_expand = FALSE)[["..."]])
+  args_matched_in_dots =
+    # positional calling args will skip over args matched by named calling args
+    args_names %in% forwarded_dots_names &
+    # extreme edge case: `epi[x]_slide(<stuff>, dot = 1, `...` = 2)`
+    args_names != "..."
+  remaining_args = args[!args_matched_in_dots]
+  remaining_args_names = names(remaining_args)
+  # note that this doesn't include unnamed args forwarded through `...`.
+  dots_i <- which(remaining_args_names == "...") # integer(0) if no match
+  n_f_args_before_dots <- dots_i - 1L
+  if (length(dots_i) != 0L) { # `f` has a dots "arg"
+    # Keep all arg names before `...`
+    mandatory_args_mapped_names <- remaining_args_names[seq_len(n_f_args_before_dots)]
+
+    if (n_f_args_before_dots < n_mandatory_f_args) {
+      mandatory_f_args_in_f_dots =
+        tail(mandatory_f_args_labels, n_mandatory_f_args - n_f_args_before_dots)
+      cli::cli_warn(
+        "`f` might not have enough positional arguments before its `...`; in the current `epi[x]_slide` call, the {mandatory_f_args_in_f_dots} will be included in `f`'s `...`; if `f` doesn't expect those arguments, it may produce confusing error messages",
+        class = "epiprocess__assert_sufficient_f_args__mandatory_f_args_passed_to_f_dots",
+        epiprocess__f = f,
+        epiprocess__mandatory_f_args_in_f_dots = mandatory_f_args_in_f_dots
+      )
+    }
+  } else { # `f` doesn't have a dots "arg"
+    if (length(args_names) < n_mandatory_f_args + rlang::dots_n(...)) {
+      # `f` doesn't take enough args.
+      if (rlang::dots_n(...) == 0L) {
+        # common case; try for friendlier error message
+        Abort(sprintf("`f` must take at least %s arguments", n_mandatory_f_args),
+              class = "epiprocess__assert_sufficient_f_args__f_needs_min_args",
+              epiprocess__f = f)
+      } else {
+        # less common; highlight that they are (accidentally?) using dots forwarding
+        Abort(sprintf("`f` must take at least %s arguments plus the %s arguments forwarded through `epi[x]_slide`'s `...`, or a named argument to `epi[x]_slide` was misspelled", n_mandatory_f_args, rlang::dots_n(...)),
+              class = "epiprocess__assert_sufficient_f_args__f_needs_min_args_plus_forwarded",
+              epiprocess__f = f)
+      }
+    }
+  }
+  # Check for args with defaults that are filled with mandatory positional
+  # calling args. If `f` has fewer than n_mandatory_f_args before `...`, then we
+  # only need to check those args for defaults. Note that `n_f_args_before_dots` is
+  # length 0 if `f` doesn't accept `...`.
+  n_remaining_args_for_default_check = min(c(n_f_args_before_dots, n_mandatory_f_args))
+  default_check_args = remaining_args[seq_len(n_remaining_args_for_default_check)]
+  default_check_args_names = names(default_check_args)
+  has_default_replaced_by_mandatory = map_lgl(default_check_args, ~!is_missing(.x))
+  if (any(has_default_replaced_by_mandatory)) {
+    default_check_mandatory_args_labels =
+      mandatory_f_args_labels[seq_len(n_remaining_args_for_default_check)]
+    # ^ excludes any mandatory args absorbed by f's `...`'s:
+    mandatory_args_replacing_defaults =
+      default_check_mandatory_args_labels[has_default_replaced_by_mandatory]
+    args_with_default_replaced_by_mandatory =
+      rlang::syms(default_check_args_names[has_default_replaced_by_mandatory])
+    cli::cli_abort("`epi[x]_slide` would pass the {mandatory_args_replacing_defaults} to `f`'s {args_with_default_replaced_by_mandatory} argument{?s}, which {?has a/have} default value{?s}; we suspect that `f` doesn't expect {?this arg/these args} at all and may produce confusing error messages.  Please add additional arguments to `f` or remove defaults as appropriate.",
+                   class = "epiprocess__assert_sufficient_f_args__required_args_contain_defaults",
+                   epiprocess__f = f)
+  }
+}
+
 ##########
 
 in_range = function(x, rng) pmin(pmax(x, rng[1]), rng[2])
diff --git a/tests/testthat/test-epi_slide.R b/tests/testthat/test-epi_slide.R
index eebcc55b9..84192f940 100644
--- a/tests/testthat/test-epi_slide.R
+++ b/tests/testthat/test-epi_slide.R
@@ -86,3 +86,14 @@ test_that("these doesn't produce an error; the error appears only if the ref tim
                      dplyr::select("geo_value","slide_value_value"), 
                    dplyr::tibble(geo_value = c("ak", "al"), slide_value_value = c(2, -2))) # not out of range for either group
 })
+
+test_that("epi_slide alerts if the provided f doesn't take enough args", {
+  f_xg = function(x, g) dplyr::tibble(value=mean(x$value), count=length(x$value))
+  # If `regexp` is NA, asserts that there should be no errors/messages.
+  expect_error(epi_slide(grouped, f_xg, before = 1L, ref_time_values = d+1), regexp = NA)
+  expect_warning(epi_slide(grouped, f_xg, before = 1L, ref_time_values = d+1), regexp = NA)
+
+  f_x_dots = function(x, ...) dplyr::tibble(value=mean(x$value), count=length(x$value))
+  expect_warning(epi_slide(grouped, f_x_dots, before = 1L, ref_time_values = d+1),
+    class = "epiprocess__assert_sufficient_f_args__mandatory_f_args_passed_to_f_dots")
+})
diff --git a/tests/testthat/test-epix_slide.R b/tests/testthat/test-epix_slide.R
index 9ef2f9afd..5eeb5c2c1 100644
--- a/tests/testthat/test-epix_slide.R
+++ b/tests/testthat/test-epix_slide.R
@@ -348,3 +348,14 @@ test_that("epix_slide with all_versions option works as intended",{
 
   expect_identical(xx1,xx3) # This and * Imply xx2 and xx3 are identical
 })
+
+test_that("epix_slide alerts if the provided f doesn't take enough args", {
+  f_xg = function(x, g) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  # If `regexp` is NA, asserts that there should be no errors/messages.
+  expect_error(epix_slide(xx, f = f_xg, before = 2L), regexp = NA)
+  expect_warning(epix_slide(xx, f = f_xg, before = 2L), regexp = NA)
+
+  f_x_dots = function(x, ...) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  expect_warning(epix_slide(xx, f_x_dots, before = 2L),
+    class = "epiprocess__assert_sufficient_f_args__mandatory_f_args_passed_to_f_dots")
+})
diff --git a/tests/testthat/test-utils.R b/tests/testthat/test-utils.R
index 08b28c97e..6648ce3ce 100644
--- a/tests/testthat/test-utils.R
+++ b/tests/testthat/test-utils.R
@@ -107,4 +107,80 @@ test_that("enlist works",{
   my_list <- enlist(x=1,y=2,z=3)
   expect_equal(my_list$x,1)
   expect_true(inherits(my_list,"list"))
-})
\ No newline at end of file
+})
+
+test_that("assert_sufficient_f_args alerts if the provided f doesn't take enough args", {
+  f_xg = function(x, g) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  f_xg_dots = function(x, g, ...) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+
+  # If `regexp` is NA, asserts that there should be no errors/messages.
+  expect_error(assert_sufficient_f_args(f_xg), regexp = NA)
+  expect_warning(assert_sufficient_f_args(f_xg), regexp = NA)
+  expect_error(assert_sufficient_f_args(f_xg_dots), regexp = NA)
+  expect_warning(assert_sufficient_f_args(f_xg_dots), regexp = NA)
+
+  f_x_dots = function(x, ...) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  f_dots = function(...) dplyr::tibble(value=c(5), count=c(2))
+  f_x = function(x) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  f = function() dplyr::tibble(value=c(5), count=c(2))
+
+  expect_warning(assert_sufficient_f_args(f_x_dots),
+    regexp = ", the group key will be included",
+    class = "epiprocess__assert_sufficient_f_args__mandatory_f_args_passed_to_f_dots")
+  expect_warning(assert_sufficient_f_args(f_dots),
+    regexp = ", the window data and group key will be included",
+    class = "epiprocess__assert_sufficient_f_args__mandatory_f_args_passed_to_f_dots")
+  expect_error(assert_sufficient_f_args(f_x),
+    class = "epiprocess__assert_sufficient_f_args__f_needs_min_args")
+  expect_error(assert_sufficient_f_args(f),
+    class = "epiprocess__assert_sufficient_f_args__f_needs_min_args")
+
+  f_xs_dots = function(x, setting="a", ...) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  f_xs = function(x, setting="a") dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  expect_warning(assert_sufficient_f_args(f_xs_dots, setting="b"),
+    class = "epiprocess__assert_sufficient_f_args__mandatory_f_args_passed_to_f_dots")
+  expect_error(assert_sufficient_f_args(f_xs, setting="b"),
+    class = "epiprocess__assert_sufficient_f_args__f_needs_min_args_plus_forwarded")
+
+  expect_error(assert_sufficient_f_args(f_xg, "b"),
+    class = "epiprocess__assert_sufficient_f_args__f_needs_min_args_plus_forwarded")
+})
+
+test_that("assert_sufficient_f_args alerts if the provided f has defaults for the required args", {
+  f_xg = function(x, g=1) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  f_xg_dots = function(x=1, g, ...) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  f_x_dots = function(x=1, ...) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+
+  expect_error(assert_sufficient_f_args(f_xg),
+    regexp = "pass the group key to `f`'s g argument,",
+    class = "epiprocess__assert_sufficient_f_args__required_args_contain_defaults")
+  expect_error(assert_sufficient_f_args(f_xg_dots),
+    regexp = "pass the window data to `f`'s x argument,",
+    class = "epiprocess__assert_sufficient_f_args__required_args_contain_defaults")
+  expect_error(suppressWarnings(assert_sufficient_f_args(f_x_dots)),
+    class = "epiprocess__assert_sufficient_f_args__required_args_contain_defaults")
+
+  f_xsg = function(x, setting="a", g) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  f_xsg_dots = function(x, setting="a", g, ...) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+  f_xs_dots = function(x=1, setting="a", ...) dplyr::tibble(value=mean(x$binary), count=length(x$binary))
+
+  # forwarding named dots should prevent some complaints:
+  expect_no_error(assert_sufficient_f_args(f_xsg, setting = "b"))
+  expect_no_error(assert_sufficient_f_args(f_xsg_dots, setting = "b"))
+  expect_error(suppressWarnings(assert_sufficient_f_args(f_xs_dots, setting = "b")),
+    regexp = "window data to `f`'s x argument",
+    class = "epiprocess__assert_sufficient_f_args__required_args_contain_defaults")
+
+  # forwarding unnamed dots should not:
+  expect_error(assert_sufficient_f_args(f_xsg, "b"),
+               class = "epiprocess__assert_sufficient_f_args__required_args_contain_defaults")
+  expect_error(assert_sufficient_f_args(f_xsg_dots, "b"),
+               class = "epiprocess__assert_sufficient_f_args__required_args_contain_defaults")
+  expect_error(assert_sufficient_f_args(f_xs_dots, "b"),
+               class = "epiprocess__assert_sufficient_f_args__required_args_contain_defaults")
+
+  # forwarding no dots should produce a different error message in some cases:
+  expect_error(assert_sufficient_f_args(f_xs_dots),
+               regexp = "window data and group key to `f`'s x and setting argument",
+               class = "epiprocess__assert_sufficient_f_args__required_args_contain_defaults")
+})